Carduoideae

Dicoma tomentosa, copyright D. Wesuls.

Belongs within: Asteraceae.
Contains: Cardueae.

The Carduoideae are a clade of composite-flowered plants supported by molecular data, members of which commonly have spiny leaves and discoid flower heads. Members include the Dicomeae, a group of African herbs, shrubs and small trees with homogamous or heterogamous flower heads that may have bilabiate peripheral florets (Panero & Funk 2002). Most have involucral bracts tapering and more or less pungent but they are obtuse or rounded in Pleiotaxis. The Tarchonantheae are a group of shrubs and trees with unisexual, dioecious capitula. They include the lowveld silver-oak Brachylaena huilensis, a dominant tree species in some eastern African woodlands. Members of the genus Oldenburgia bear capitula solitarily or in few-headed cymes whereas in Tarchonanthus and Brachylaena they are numerous.

Thistle be the one
Published 9 November 2009
Cardoon Cynara cardunculus with humans to scale, from here.

The composite-flowering plants of the Asteraceae are one of the largest (23,000 species, according to Wikipedia) and most distinctive plant groups out there—even a complete botanical dunce like yours truly can usually recognise an example of Asteraceae. Asteraceae include such plants as daisies and chrysanthemums in which the “flower” is in fact a large number of tiny flowers all pressed together, hence the old name for the family of “Compositae”. Different authors have proposed different classifications within Asteraceae over the years, but twelve subfamilies were recognised by Panero & Funk (2008). The subfamily Carduoideae as recognised by these authors includes the three tribes Dicomeae, Tarchonantheae and Cardueae (earlier authors had used the name to cover a broader paraphyletic assemblage, or restricted it to include only Cardueae). The genus Oldenburgia may be included in Tarchonantheae or it may be placed in its own separate tribe (Funk et al. 2009). No unique morphological features characterise this subfamily (though most species have a ring of papillae on the style underneath the stigmatic branches), but it is well supported molecularly.

The tribes Dicomeae and Tarchonantheae are primarily found in Africa and Madagascar (two species of Dicomeae and one of Tarchonantheae are found in Asia). The seventeen species of Tarchonantheae (including Oldenburgia) are all shrubs or trees; the 75–100 species of Dicomeae include herbs, shrubs and trees. Tarchonantheae includes the genus Brachylaena, species of which predominate in southern African and Madagascan woodlands. Brachylaena species are noted for producing dense, high quality wood, and are also among the largest of the Asteraceae, reaching 40 m in height (Beentje 2000).

Brachylaena discolor from southeastern Africa, from here.

The largest by far of the three tribes is the Cardueae*, the thistles, with some 2500 species distributed through Eurasia from the Mediterranean to central Asia. The majority of Cardueae are herbs, though there are a few small shrubs or even small trees in the tribe. Most members of Cardueae have distinctive discoid flower heads** and, of course, many have spiny leaves.

*I have just been through the painful, arduous and not-entirely-productive process of trying to decide whether ‘Cardueae’ or ‘Cynareae’ is the correct name for this tribe; both names are used regularly. Lamarck & de Candolle published the name ‘Cynarocephalae’ in 1806 (Reveal 1997); Cardueae was published by Cassini in 1819 (Solbrig 1963). The question therefore hinges on whether the ‘-cephalae’ in Cynarocephalae represents a suffix like ‘-idae’ or ‘-aceae’ or whether the name is descriptive of plants with ‘heads like Cynara‘; if the former, Cynareae has priority from 1806; if the latter, Cynareae was not published until 1830 (and illegitimately so at that) and Cardueae has priority. Botanists still seem to be in the process of duking out which interpretation is corrent, and I suspect that it may take the ICBN stepping in to settle the matter.

**Composite flower heads may contain both ‘ray’ and ‘disk’ florets (the little individual flowers). If you think of a daisy, the ‘ray’ florets are the ones around the edge that carry the large petals while the ‘disk’ florets are the central ones without petals. Discoid flower heads like those of Cardueae contain only disk florets and no ray florets.

Side view of flower head of Atractylis cancellata, a Mediterranean thistle species in which the rosette of (particularly evil-looking) leaves around the flower head curls upwards to surround it. Photo by Manuel Ramos.

Species of Cardueae most often bring themselves to humanity’s attention through the fact that a number of them are significant weed species, and very few Cardueae are regarded with any sort of affection. The Scotch thistle Onopordum acanthium is of course popular in Scotland where it is the national flower; according to legend, a Scottish encampment was saved from a sneak attack by Vikings when one of the invaders yelled out after stepping on a thistle, alerting the sentries to their presence. Also granted a certain regard is Cynara cardunculus, the cardoon/globe artichoke. Earlier classifications recognised two species, the cardoon C. cardunculus grown for its edible stalks and the artichoke C. scolymus grown for its similarly edible flower heads, but there is no doubt that the latter is a horticulturally derived variety of the former. Perhaps the best demonstration of this is that escaped seeds from artichoke fields in California and Australia have given rise to wild populations of ‘cardoons’ (Sonnante et al. 2007). I will also note that artichokes would also be a feature of my ideal garden—not because I’m a fan of eating artichokes (I think they’re pretty tasteless) but because these two-metre tall thistles are such spectacular plants.

Systematics of Carduoideae
<==Carduoideae
|--+--CardueaePF02
| `--TarchonantheaePF02
| |--OldenburgiaPF02
| `--+--TarchonanthusPF02
| `--BrachylaenaPF02
| |--B. glabraK02
| `--B. huillensisR-S02
`--DicomeaePF02
|--ErythrocephalumPF02
|--GladiopappusPF02
|--MaclediumPF02
|--CloiseliaPF02
|--PasaccardoaPF02
|--PleiotaxisPF02
`--Dicoma Cassini 1817PF02
|--D. cuneneensisCV06
|--D. dinteriCV06
|--D. obconicaCV06
`--D. tomentosaPP07

*Type species of generic name indicated

References

Beentje, H. J. 2000. The genus Brachylaena (Compositae: Mutisieae). Kew Bulletin 55 (1): 1–41.

[CV06] Craven, P., & P. Vorster. 2006. Patterns of plant diversity and endemism in Namibia. Bothalia 36 (2): 175–189.

Funk, V. A., A. Susanna, T. F. Steussy, & H. E. Robinson. 2009. Classification of Compositae. In: Funk, V. A., A. Susanna, T. F. Stuessy & R. J. Bayer (eds) Systematics, Evolution, and Biogeography of Compositae pp. 171–189. International Association for Plant Taxonomy (IAPT): Vienna.

[K02] Krüger, M. 2002. Revision of the Afrotropical Ennominae of the Drepanogynis group IV: the genus Drepanogynis Guenée (Lepidoptera: Geometridae). Transvaal Museum Monograph 13: 1–220.

[PF02] Panero, J. L., & V. A. Funk. 2002. Toward a phylogenetic subfamilial classification for the Compositae (Asteraceae). Proceedings of the Biological Society of Washington 115 (4): 909–922.

Panero, J. L., & V. A. Funk. 2008. The value of sampling anomalous taxa in phylogenetic studies: major clades of the Asteraceae revealed. Molecular Phylogenetics and Evolution 47 (2): 757–782.

Reveal, J. L. 1997. Early suprageneric names in Asteraceae. Compositae Newsletter 30: 29–45.

[R-S02] Russell-Smith, A. 2002. A comparison of the diversity and composition of ground-active spiders in Mkomazi Game Reserve, Tanzania and Etosha National Park, Namibia. Journal of Arachnology 30 (2): 383–388.

Solbrig, O. T. 1963. Subfamilial nomenclature of Compositae. Taxon 12 (6): 229–235.

Sonnante, G., A. V. Carluccio, R. Vilatersana & D. Pignone. 2007. On the origin of artichoke and cardoon from the Cynara gene pool as revealed by rDNA sequence variation. Genetic Resources and Crop Evolution 54 (3): 483–495.

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